The invention relates to polyolefin resin blends having superior physical properties including enhanced scratch resistance, rigidity, and toughness. The invention also relates to the process of manufacturing such polyolefin resin blends and to articles produced from such polyolefin resin blends.
In numerous applications, such as many in the automotive industry, a polymeric material that exhibits a good level of scratch resistance balanced with rigidity and impact toughness is desired. These properties tend to vary, however, such that efforts to enhance one will often result in deterioration of one, or both, of the other beneficial properties.
Polypropylene blends are useful in a wide variety of applications due to their strength, environmental resistance, and processability. While highly crystalline polypropylene does exhibit good mar and scratch resistance, it does not possess the impact toughness required in many important applications such as the making of automobile parts. Special polymeric materials have been developed that overcome this problem to some degree.
Attempts to remedy polypropylene""s deficiency in impact toughness by blending with impact modifying copolymers of ethylene and other alpha-olefins, terpolymers of ethylene, other alpha-olefins, and dienes have not been completely successful. Elastomer modified polypropylene blends, also known as thermoplastic polyolefins, have the advantage of improved toughness, especially for cold temperature impact. They are widely used for formed or shaped articles, such as automotive parts, toys, furniture, and housing products. Although the impact toughness of those compositions is improved by these modifiers, the scratch resistance has been found to decrease. That is, the scratch resistance of polypropylene blends containing impact modifiers, such as ethylene-propylene copolymers, ethylene-propylene terpolymers, ethylene-butene copolymers, or ethylene-octene copolymers, is poor.
Increasing the crystallinity of the polypropylene to obtain a harder surface, and/or adding hard mineral filler to these blends, has been attempted as a countermeasure but without complete success. One conventional method to enhance surface characteristics is to use inorganic particulate material. Uniform dispersion of these particulates is difficult to achieve, however, and this tends to result in non-uniform surface properties in such products. The use of these particulates also tends to damage other desirable physical properties of the polyolefin, resulting in loss of impact strength and/or toughness. Debonding of such particulates from the polyolefin system also contributes to stress whitening, which is not desirable.
Another conventional way to enhance surface characteristics of various articles is to apply acrylic polymers or coatings to an article and subsequently cure the polymer or coating with a radiation source, such as ultraviolet radiation.
A method to enhance surface characteristics of polyolefins is described in U.S. Pat. No. 4,000,216, which discloses an extrudable, moldable, or heat formable blend of a thermoplastic polymer and a surface altering agent of at least one monoethylenically unsaturated monomer for said thermoplastic polymer, wherein the surface altering agent has cross-linked polymer particles having an average size of 1 to 30 microns. The surface altering agent is preferably prepared by an endopolymerization, which is used with a compatible polyolefin to be altered.
U.S. Pat. No. 5,880,198 describes thermoplastic resin compositions comprising polypropylene, a styrene containing elastomer, and talc, and having an acceptable balance of toughness and rigidity. The disclosure stresses the importance of the proportions of each component used to achieve this balance. In U.S. Pat. No. 6,384,122, a similar balance of properties is reported for a thermoplastic resin comprising an ethylene-propylene based polymer composition, an ethylene alpha-olefin copolymer based rubber and/or a rubber containing vinyl aromatic compounds and talc. Another example of a composition that provides an acceptable balance of toughness and rigidity is reported in Japanese Patent Application 10219040A for a resin composition consisting of a polyolefin based resin and a block copolymer based on aromatic vinyl and butadiene monomer units. Polymer blends which can be formed or shaped into lightweight and durable articles useful, for example, as automobile parts, toys, housings for various types of equipment, and the like, are well known in the art.
The physical and/or chemical properties of the thermoplastic polyolefin blends can be modified either by blending them with other thermoplastic polymers, or by incorporating into them materials having one or more polar groups, or both. For example, U.S. Pat. No. 4,946,896 describes a thermoplastic polyolefin comprising 20-80 weight percent polypropylene; 5-38 weight percent of an ethylene copolymer consisting of ethylene, an ester unit of either alkyl acrylate or methacrylate, and an unsaturated dicarboxylic acid anhydride; and 5-70 weight percent ethylene-propylene rubber. Similarly, U.S. Pat. No. 4,888,391 describes a polyolefin composition comprising a blend of a polyolefin as the continuous phase with an ethylene/acrylate/acrylic acid terpolymer as a discontinuous phase. These polyolefin-based blends are paintable. Another example of improving both scratch and impact resistance is reported in U.S. Pat. No. 6,423,779, where polypropylene and a polyphenylene oxide resin are blended together with a compatibilizer.
Despite these prior art formulations, there remains a need to obtain polymeric materials that have a good level of mar/scratch resistance along with the physical property requirements of rigidity, strength, processability, and low temperature impact toughness. The present invention provides certain blends that meet these needs.
The present invention relates to a polyolefin resin blend comprising: from about 10 to 90 weight percent of a semi-crystalline polyolefin; from about 0.1 to about 50 weight percent of a propylene-based polyolefin-metal salt, from about 0.1 to about 50 weight percent of a styrenic block ionomer. The propylene-based polyolefin-metal salt is advantageously a reaction product of a propylene-containing homopolymer or copolymer and at least one organic monomer containing at least one hydrophilic moiety. Furthermore, the reaction product is at least partially neutralized with at least one metal ion.
The styrenic ionomer is typically a styrenic block ionomer, although in preferred embodiments it can include mixtures thereof with a polymeric metal salt component. The optional but preferable polymeric metal salt is typically a reaction product of (a) a random styrenic copolymer which includes at least one unsaturated dicarboxylic acid derivative incorporated into the styrene backbone; and (b) at least one metal ion present in an amount sufficient to at least partially neutralize each type of unsaturated dicarboxylic acid derivative of (a). Also, the styrenic block ionomer is preferably a reaction product of a styrenic block copolymer modified with at least one polar group, the reaction product being at least partially neutralized with at least one metal ion. Each metal ion used to at least partially neutralize the reaction products disclosed herein independently includes lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc, aluminum or mixtures thereof.
The polyolefin resin blend preferably includes the semi-crystalline polyolefin in an amount of from about 15 to about 85 weight percent, the propylene-based polyolefin-metal salt in an amount from about 1 to about 45 weight percent, and the styrenic block ionomer in an amount of from about 1 to about 45 weight percent of the polyolefin resin blend. Preferably, the semi-crystalline polyolefin is present in an amount of from about 20 to about 80 weight percent, the propylene-based polyolefin-metal salt is present in an amount of from about 2 to about 40 weight percent, and the styrenic block ionomer is present in an amount of from about 2 to about 40 weight percent of the polyolefin resin blend.
The semi-crystalline polyolefin preferably includes: one or more of homopolymers of propylene, homopolymers of ethylene, copolymers of propylene and a C2 to C20 alpha-olefin component, copolymers of ethylene and a C3 to C20 alpha-olefin component, or mixtures thereof. Also, it is useful for each hydrophilic moiety and each polar group to each independently include an ethylenically unsaturated carboxylic acid or carboxylic anhydride monomer, and preferably methacrylic acid, acrylic acid, maleic anhydride, or a mixture thereof
The styrenic block ionomer, typically a styrenic block copolymer modified with at least one polar group, preferably includes a styrenic block segment and an elastomeric block segment that is optionally hydrogenated. This includes bocks of styrene-ethylene/butylene, styrene-ethylene/butylene-styrene, styrene-ethylene/propylene, styrene-ethylene/propylene-styrene, styrene-ethylene/propylene-styrene-ethylene-propylene, styrene butadiene, styrene-butadiene-styrene, styrene-butylene-styrene, styrene-butylene-butadiene-styrene, styrene-isoprene-styrene, or combinations thereof.
When included, the random styrenic copolymer of the polymeric metal salt preferably includes styrene-maleic anhydride. In one preferred embodiment, the polyolefin resin blend further includes an impact modifier in an amount from about 0.1 to about 40 weight percent. Preferably, the impact modifier is present in an amount from about 1 to about 20 weight percent and comprises a styrenic block copolymer, an hydrogenated styrene butadiene random copolymer, or mixtures thereof. If desired, a mineral filler can be included in an amount from about 1 to about 40 weight percent. The mineral fillers are generally present in an amount from about 5 to about 25 weight percent and preferably include talc, calcium carbonate, wollastonite, alumina trihydrate, barium sulfate, calcium sulfate, carbon blacks, metal fibers, boron fibers, ceramic fibers, polymeric fibers, kaolin, glass, ceramic, carbon or polymeric microspheres, silica, mica, glass fiber, carbon fiber, clay, or mixtures thereof.
The invention also relates to an article made of one of the polyolefin resin blends disclosed herein.
Furthermore, the invention relates to a method of preparing an article from a polyolefin resin blend which includes: melt blending a semi-crystalline polyolefin resin component, a propylene-based polyolefin-metal salt and from a styrenic block ionomer component of the types described above so as to form a blend, wherein the melt blending is at a sufficiently high temperature so that each polymer is at least partially melted; and molding the blend into an article having a surface, wherein the surface of the article when subjected to a 20 Newton load has less than about a 3 on the scratch rating scale and has no ribbing or tearing.